This application claims priority of German Patent Application No. 10135994.2, filed Jul. 24, 2001, the entire contents of which are hereby incorporated herein by reference.
The present invention generally relates to a method for executing a computer program for post-processing computer tomography images. In particular, it relates to a method for executing a computer program for image post-processing for reducing line artifacts that are caused by high-density structures in computer tomography images.
If in the course of a computer tomographic (CT) examination, high-density objects are located in the scanning plane of the computer tomograph, the x-rays are absorbed so strongly by these objects that zones located in the shadow of these objects are now only transradiated by very weak x-radiation. Owing to the weak radiation, the signal-to-noise ratio is worsened for the examination regions in the shadow of the high-density objects, such that the values of the absorption coefficients for these regions can be determined only with relatively high uncertainty.
Given the presence of such high-density objects, it follows that artifacts, that is to say structures with nothing corresponding to them in the original, are to be observed in the CT images. Starting from the images of the high-density objects, these artifacts propagate like rays over the CT image. The image of such a high-density object in a CT image is denoted below as interfering object. In addition to metal objects such as, for example, belt buckles, metal clips or the like, it is frequently also tooth fillings or bone structures that are the cause of the described ray-shaped artifacts. Because of their ray-shaped nature, these artifacts are denoted as line artifacts. In the case of bone structures, the starting point of these line artifacts is formed in general by bone margins.
The cause of the line-like and ray-shaped course of the artifacts presides in the type of scanning of an original, to be examined, for creating a CT image.
The artifacts can sometimes determine the image to such an extent that they strongly impair, or even sometimes render impossible, a diagnostic analysis of the image. The attempt to eliminate the cause of the described line artifacts in the raw data supplied by a computer tomography examining instrument presupposes accurate knowledge of the imaging geometry of the examining instrument, and an enormous analytical outlay for minimizing the artifacts.
It is therefore an object of an embodiment of the present invention to specify a simple method and a computer software product for reducing line artifacts in CT images.
An object may be achieved by a correction method for reducing artifacts caused by structures of high x-ray absorption in computer tomography images that has steps for identifying an interfering object in a computer tomography image, for locating the interfering object in the computer tomography image, and for filtering the computer tomography image as a function of the position of selected pixels relative to the interfering object.
An object may be further achieved by a computer software product for reducing artifacts, caused by structures of high x-ray absorption in computer tomography images, for executing the above-described method on a data processing system with programmed instructions for executing on the data processing system.
A method according to an embodiment of the invention neither requires information on special structural features of the computer tomography examining instrument, nor is it dependent on the raw data created by a measurement. It advantageously reduces the artifacts in the finally reconstructed CT images.
An interfering object in a computer tomography image can be advantageously located by calculating the position of the centroid of the interfering object in this computer tomography image, such that the interfering object can be treated as a punctiform entity. Furthermore, to obtain improved filtering of a computer tomography image, it is advantageous to use all the pixels outside an interfering object for the purpose of filtering the computer tomography image. The filtering preferably extracts the instances of interference from the computer tomography image such that the corrected computer tomography image is advantageously obtained from the difference between the original computer tomography image and the filtered computer tomography image.
In accordance with an advantageous development, a computer tomography image can be filtered in two steps, in which case it is only neighboring pixels of a pixel to be filtered that are located on a circular arc around the centroid of the interfering object and through the pixel to be filtered that are included in a first filter-step, and it is only neighboring pixels of a pixel to be filtered that are located on the straight line through the pixel and the centroid of the interfering object that are included in a second filter step. It is thereby possible to make use transverse to the rays of the line artifacts of different filter criteria than in the direction of these line artifacts.
In order to be able to treat more simply complex artifact structures that emanate from extended interfering objects, large interfering objects are preferably split up into a multiplicity of smaller interfering objects.
It is advantageous to assign an interfering object only pixels with a gray-scale value or a color value that corresponds to an x-ray absorption value of at least 2000 Hounsfield units or more, such that the extent of the interfering objects in a CT image corresponds to the image of a corresponding high-density object.
Furthermore, in an advantageous way, given the presence of a plurality of interfering objects in a CT image, the interfering objects may be weighted in accordance with their x-ray absorption value for the correction method. The interfering objects may be sorted in accordance with the weighting, and starting with the first interfering object of the sorting sequence, the CT image for each individual one of the interfering objects may be corrected sequentially in accordance with the sorting sequence, a computer tomography image corrected with reference to a first interfering object of the sorting sequence forming the output for a further correction with reference to a second interfering object of the sorting sequence. In this case, the interfering objects are preferably sorted downward from a higher degree of x-ray absorption to a lower degree of x-ray absorption. Undertaking corrections of the instances of interference, caused by line artifacts in a CT image, separately for each individual interfering object permits the artifacts caused by the individual interfering objects to be eliminated specifically and individually, the largest line artifacts being reduced first by firstly performing the filtering of the CT image for interfering objects with a high degree of x-ray absorption.
It is possible with further advantage to restrict the number of the interfering objects to be used for filtering, such that only line artifacts that contribute substantially to the interference of the CT image are reduced.
The correction method according to an embodiment of the invention and the computer software product according to an embodiment of the invention for reducing artifacts caused by structures of high x-ray absorption in CT images can be applied for the purpose of image evaluation in computer tomography examining instruments and/or in systems for medical image processing. In particular, embodiments of the invention can be used in radiological diagnostics, for example for orthodontic examinations in which tooth fillings frequently cause line artifacts in the CT images. If these are not reduced, they greatly diminish the diagnostic value of a CT image.